1. Field of the Invention
The present invention relates generally to directional thermal neutron detectors, and more particularly pertains to a directional thermal neutron detector which utilizes the inherent angular response of large area, planar silicon detectors and gadolinium foils to determine the direction of a thermal neutron radiation source. The directional thermal neutron detector can also advantageously utilize the shadowing provided by adjacent thermal neutron detector modules, which are positioned to function as shields, to enhance the directional response thereof.
2. Discussion of the Prior Art
Nuclear weapon non-proliferation and counter-proliferation have become national priorities, and various nuclear non-proliferation sensors, thermal neutron detectors, and radiation level monitors are required to conduct surveillance and inspection of sites, nuclear power plants, space experiments, etc. in nuclear non-proliferation and treaty-verification programs.
Thermal neutron detection offers an effective method for determining the presence of spontaneously fissionable materials which are used in nuclear weapons. Fission neutrons emitted from such weapons-related materials undergo collisions with their surroundings and readily become "thermalized". Since the natural background of thermal neutrons is very low at the earth's surface (.sup..about. 1 neutron/s per 1000 cm.sup.2), detection of thermal neutrons at a rate significantly above this level is a cause for suspicion of the presence of fissionable nuclear materials. Accordingly, thermal neutrons can be detected to determine the presence and location of fissionable nuclear materials and nuclear weapons.
Conventional portable neutron survey meters are not suitable for low level thermal neutron detection applications because their counting sensitivity is very poor. Only large, unmoderated .sup.3 He proportional tubes can obtain a comparable thermal neutron sensitivity for a limited available area. .sup.3 He tubes, however, cannot easily furnish a source directionality measurement that is unique to the present detector system.
The thermal neutron detector of the present invention offers unique advantages relative to prior art detection systems based upon BF.sub.3 or .sup.3 He proportional counters that are commonly used for neutron detection. It provides comparable neutron sensitivity, and does not require high voltage for operation. The present detector system avoids problems inherent in field deployment of high voltage equipment, such as break down and sparking in a humid environment. The present detector system also has a more compact and rugged design for improved reliability under vibration and mechanical shock. The thermal neutron detector of the subject invention is highly modular and, therefore, less susceptible to single point failures, while systems of proportional counters of comparable area (e.g., 5 or 6 one inch tubes) would suffer greater loss of efficiency if one or more proportional tubes failed. Furthermore, the thermal neutron detector of the present invention provides a directional detecting capability, a feature which is not easily implemented in a portable system using proportional tubes. Existing thermal neutron detectors require special collimation to achieve a directional sensing capability, which results in the addition of weight to the system.
Radiation monitors for thermal neutrons based upon thin gadolinium foils coupled with silicon detectors have been in use in the prior art for several years. Recently the availability of large area silicon photodiodes makes large area monitors feasible, with a potential for arrays of such detectors with active areas well in excess of 100 cm.sup.2. In practice, however, there are limitations to the area of a single detector element and its associated pulse processing electronics. The noise levels in the photodiode and preamplifier system must be sufficiently low such that the low energy (29-200 keV) conversion electrons emitted by thermal neutron capture in gadolinium are detected with sufficient efficiency to obtain a high area-efficiency (A.epsilon.) product for the monitor. For large area silicon detectors, the capacitance of the parallel elements in the detector can reach several hundred picofarads which becomes the dominant factor in the noise of the system, and determines the required low level discriminator threshold setting. As the discriminator threshold is increased, the intrinsic detection efficiency is reduced, and the result is a tradeoff between increased detection area and reduced efficiency.